The present invention relates to a recording paper which is partially pressure-sensitized and a process for preparing thereof, and more in detail, relates to a recording paper which is partially pressure-sensitized obtained by retaining microcapsules containing a solution of a colour former on a specified part of a surface of a sheet material by the use of a binder, particularly a photosetting binder.
In recent years, the recording papers for computer or businessform such as slips for business have come to be more and more complicated and diversified in accordance with the improvement of business efficiency and mechanization of business, and a considerable number of the businessforms take the form including a plurality of copying sheets.
Although in such cases, the pressure sensitive recording paper is frequently used, the conventional pressure sensitive recording paper is made by piling an upper sheet of paper (hereinafter referred to CB sheet) having the back surface thereof painted with microcapsules containing a solution of the so-called leucotype dyestuff (as a colour-former) as the core substance onto a lower sheet of paper (hereinafter referred to as CF sheet) having the front surface thereof painted with acid clay or an acidic resin (as a developer), and in the actual use, by the application of writing pressure by a pen or printing pressure by a typewriter, the microcapsules at the pressed part(s) are broken to contact the colour former with the developer, thereby causing coloration and resulting in printing of the handwritten or typewritten items. In the case of pressure sensitive recording paper having a number of copying sheets, one or more sheets of paper each having both the front surface thereof painted with a developer and the back surface thereof painted with a colour former (hereinafter referred to as CBF sheet(s)) is or are inserted between the CB sheet and the CF sheet, and the thus prepared multi-layered paper is used for the purpose.
However, since the microcapsules have been painted on the whole surface of the sheet material, it is necessary to desensitize the developer on the part(s) of the surface of the sheet materrial by the use of a reducer ink (de-sensitizing ink) or the like in the cases where only a specified number of the sheets of recording paper should be copied or only specified part(s) of a slip should be copied, for preventing the unnecessary copying. Such a technique would require much labor and it is inevitable to waste the microcapsules on the part(s) not to be copied or on the number of sheets of copying paper by the de-sensitizing.
In view of the above-mentioned demerit of the conventional pressure sensitive recording paper, it is considered that a recording paper which is partially pressure-sensitized can be obtained without using any de-sensitizing ink, if it is able to retain the microcapsules only on the really necessary part of the surface of the DB sheet. Namely, in the case where a pressure sensitive recording paper having the microcapsules painted only on the necessary part(s) of the surface thereof can be prepared by a spot-printing method or the like, a really epoch-making pressure sensitive recording paper can be offered.
However, since at present the conventional pressure sensitive recording paper is prepared by painting a surface of base paper with an aqueous slurry-like material comprising a water-soluble binder, water as a solvent, adjuvants and the microcapsules, it is practically almost impossible to apply such a technique mentioned above to the spot-printing and the like, because the thus spot-printed sheet of paper partly wrinkles on drying.
On the other hand, from the economical viewpoint of not using the expensive microcapsules and the technical merits of copy-printing only on the necessary part of the surface, the development of the recording paper which is partially pressure-sensitized has been eagerly requested.
In answering the request, the processes for preparing the recording paper which is partially pressure-sensitized have been developed. As an example, a process wherein the microcapsules are dispersed in an organic solvent containing a vehicle and the thus obtained organic dispersion is painted only on the specified part(s) of the surface of sheet material by a printing machine of photogravure type or flexo type has been known.
However, on the recording paper which is partially pressure-sensitized obtained by the process of painting an organic dispersion of microcapsules in a solution of a vehicle on a sheet of paper and drying the thus painted paper, the whole surface of the microcapsule is covered by a binder and the microcapsules are fixed onto the surface of the sheet of paper and accordingly, the breaking efficiency of the microcapsules and the transfer efficiency from CB sheet to CF sheet are inhibited resulting in the necessity of painting a large amount of the microcapsules for obtaining a predetermined color density.
As a result of the studies of the present inventors concerning the above-mentioned several problems, it has been found that by a process comprising the steps of painting a binder on a specified part(s) of the surface of a sheet material, before solidifying of the thus painted binder, adhering the microcapsules only onto the thus specified part(s) of the surface where the binder has been painted and solidifying the binder, the recording paper which is partially pressure-sensitized showing an extremely high colouring density is obtained and based on the finding, the present inventors have attained the present invention.